Determination of the significance of atomic concentration on surface properties of Ba x Mg1- x F2 alloy coatings via microscopic and spectroscopic techniques

Abstract

Both BaF₂ and MgF₂ compounds and Ba _x Mg_1-x F₂ alloy thin films were deposited on glass and silicon (Si) substrates in nanometric sizes (100 ± 10 nm) in a high vacuum environment by radio frequency (rf) magnetron sputtering. Using BaF₂ (99% purity) and MgF₂ (99% purity) target materials and adjusting the power levels applied to these targets, Ba _x Mg_1-x F₂ alloy coatings at different atomic concentrations were formed under the same vacuum conditions. The microstructure and surface characteristics of the samples were analysed with the help of spectroscopic and microscopic methods. For the surface characterization, with scanning acoustic microscopy (SAM), 2-dimensional surface acoustic images of the samples were mapped, the surface acoustic impedance values were determined, and information about the micro hardness of the materials was obtained. Surface roughness values and grain sizes were obtained by taking 3-dimensional surface images of investigated materials using atomic force microscopy (AFM). Average nanometric particle sizes were determined for each sample with scanning electron microscopy (SEM), therefore, information about surface homogeneity was obtained. For the microstructural characterization, quantitative elemental analysis was performed using scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM-EDS), and stoichiometric ratios of atomic compositions were identified. By evaluating the data obtained from the microscopic and spectroscopic measurements, the effect of the atomic concentration parameter on the morphological properties of the material was determined. The usability of the produced binary fluoride alloy thin film coatings is promising for emerging optoelectronic, ceramic industry, biomedical and surface acoustic wave applications.

Fig. 1. AFM images of samples coated on quartz glasses in 10 × 10 μm² scanning areas: (a) BaF₂, (b) MgF₂, (c) Ba_xMg_1−xF₂ (x > 0.5) and (d) Ba_xMg_1−xF₂ (x < 0.5).

Fig. 2. Surface images of the BaF₂ compound obtained by SEM, scale bars: (a) 200 nm, (b) 1 μm.

Fig. 3. Surface images of the MgF₂ compound obtained by SEM, scale bars: (a) 200 nm, (b) 1 μm.

Fig. 4. Surface images of Ba_xMg_1−xF₂ (barium-rich, x > 0.5) alloy obtained by SEM, scale bars: (a) 200 nm, (b) 1 μm.

Fig. 5. Surface images of Ba_xMg_1−xF₂ (magnesium-rich, x < 0.5) alloy obtained by SEM, scale bars: (a) 200 nm, (b) 1 μm.

Fig. 6. SEM-EDS analysis results of 100 nm thick BaF₂ compound. (a) SEM image of the surface topography, EDS mappings of (b) Ba, (c) F elements in BaF₂ coating deposited on silicon substrate, (d) EDS spectrum of the area shown in (a) (scale bar is 2 μm). F element's and Ba element's EDS peaks are represented by purple and green arrows respectively.

Fig. 7. SEM-EDS analysis results of 100 nm thick MgF₂ compound. (a) SEM image of the surface topography, EDS mappings of (b) Mg, (c) F elements in MgF₂ coating deposited on silicon substrate, (d) EDS spectrum of the area shown in (a) (scale bar is 2 μm). F element's and Mg element's EDS peaks are represented by purple and orange arrows respectively.

Fig. 8. SEM-EDS analysis results of 100 nm thick Ba_xMg_1−xF₂ (barium-rich, x > 0.5) alloy. (a) SEM image of the surface topography, EDS mappings of (b) Ba, (c) Mg, (d) F elements in Ba_xMg_1−xF₂ (barium rich, x > 0.5) coating deposited on silicon substrate, (e) EDS spectrum of the area shown in (a) (scale bar is 2 μm). F element's, Ba element's, and Mg element's EDS peaks are represented by purple, green, and orange arrows respectively.

Fig. 9. SEM-EDS analysis results of 100 nm thick Ba_xMg_1−xF₂ (magnesium-rich, x < 0.5) alloy. (a) SEM image of the surface topography, EDS mappings of (b) Ba, (c) Mg, (d) F elements in Ba_xMg_1−xF₂ (magnesium-rich, x < 0.5) coating deposited on silicon substrate, (e) EDS spectrum of the area shown in (a) (scale bar is 2 μm). F element's, Ba element's, and Mg element's EDS peaks are represented by purple, green, and orange arrows respectively.

Fig. 10. Two-dimensional acoustic impedance maps of (a) BaF₂ compound, (b) MgF₂ compound, (c) Ba_xMg_1−xF₂ alloy (x > 0.5) and (d) Ba_xMg_1−xF₂ alloy (x < 0.5), recorded by 80 MHz SAM. The scanning area is 4.8 mm × 4.8 mm with 300 × 300 sampling points. The colour bar represents the variation in acoustic impedance values.